To take an approach to the low-carbon society, an increasing need for careful management of energy exists in the electric/electronic, transportation and other fields. This requires more precise control of a system which is, in turn, equipped with more than ever electronic components. In fact, the automotive field, for example, has the trend that the proportion of hybrid vehicles, plug-in hybrid vehicles, and electric vehicles replacing gasoline vehicles is increasing in the market. These hybrid and electric vehicles must be loaded with motors, inverters, batteries, and other electronic components which are unnecessary for gasoline vehicles. Since these electronic components generate heat during operation, efficient heat dissipation is essential to insure normal operation of the components. Accordingly, the heat-dissipating materials become of greater importance.
More than ever electronic components must be mounted within a limited space, indicating that electronic Components are kept under widely varying conditions including temperature, mount angle, etc. Under the circumstances, heat-generating electronic components and heatsinks are not always held horizontal and accordingly, a heat-conductive material connecting them is often mounted at a certain angle. In such a service environment, a heat-conductive silicone adhesive material, heat-conductive potting material, or RTV heat-conductive silicone rubber composition is used in order to prevent the heat-conductive material from sagging and falling out of the space between the heat-generating component and the heatsink, as disclosed in JP-A H08-208993, JP-A S61-157569, and JP-A 2004-352947. However, all these heat-conductive materials form a complete bond to members and undesirably lack re-workability. Since the heat-conductive material becomes very hard after bonding, it cannot withstand repeated stresses induced by thermal strain and separates apart from the heat-generating component to allow for the entry of air, leading to a ramp of thermal resistance.
The above problem can be solved by a one package addition cure heat-conductive silicone composition as disclosed in JP-A 2002-327116. This composition remains re-workable and anti-sagging even after heat curing, and the cured composition is soft enough to play the role of a stress relief agent. Nevertheless, this heat-conductive material suffers from several problems. For example, it must be stored in a refrigerator or freezer and thawed prior to use. In applying the heat-conductive material, it must be heated and cooled. Then the manufacturing system must be equipped with a heating/cooling oven. The heating and cooling steps take a long time, leading to a reduction of manufacturing efficiency. From the standpoint of energy efficiency, these steps are inefficient because not only the heat-conductive material, but also an overall component must be heated. Additionally, there is a potential risk that if any cure inhibitor is present on the coating surface, the heat-conductive material remains under-cured even when heated.
To obviate the cumbersome handling of heat-conductive material including refrigeration/thaw management for storage and heating/cooling steps for application, JP-A 2003-301189 proposes a one package addition cure heat-conductive silicone composition which has been heat crosslinked during preparation. This heat-conductive material has overcome the above-discussed problems, but the tradeoff is that it has too high a viscosity to coat. There are problems that heavy loading of filler is difficult due to the high viscosity of the base polymer and the manufacture process involving crosslinking reaction takes a long time.
It would be of significance to have a heat-conductive silicone grease composition which is amenable to coat at the initial, thereafter increases its viscosity at room temperature rather than curing at room temperature so that it remains flexible, easy to re-work and anti-sagging, does not need refrigeration or freezing for storage, does not need heating upon application, avoids any undesired viscosity buildup, is easy to manufacture, allows for heavy loading of filler, and offers high heat transfer.
CITATION LIST                Patent Document 1: JP-A H08-208993        Patent Document 2: JP-A S61-157569        Patent Document 3: JP-A 2004-352947 (US 2004242762, DE 102004025867, CN 100374490)        Patent Document 4: JP-A 2002-327116 (EP 1254924 B1, U.S. Pat. No. 6,649,258)        Patent Document 5: JP-A 2003-301189 (EP 1352947 A1, U.S. Pat. No. 6,818,600)        